A signal storage device capable of providing stable long term hold characteristics has important applications to control systems. In a closed loop system, for example, error signals are produced signifying the offsets of the closed loop system from a reference position. The reference position may be defined as the desired position of the system and thus, the error signals will be reduced to zero if the system achieves this reference position. In many cases, if the error signals corresponding to these offsets can be accumulated and stored without drift due to temperature, etc., the required time between adjustments or offset nulling can be greatly increased.
It is known that feedback control loops are often constrained to generate a maximum instantaneous error signal during each processing cycle notwithstanding the fact that the actual system offset from its reference position in terms of a true error signal is greater that the maximum error signal which can be generated by the feedback control loop due to limited instantaneous dynamic range. The reasons for this limitation to the maximum instantaneous error signal are various and a discussion of these reasons is not necessary for a full understanding of the invention described hereinafter. It is sufficient to understand that the limitation does exist. With an actual error greater than the maximum instantaneous error signal which can be generated during a processing cycle, the system reference point can never be reached during a single processing cycle. If the system offset from its reference position is not caused to increase by reason of any external influence, such as the physical movement of the system being controlled, the system can achieve zero error over several processing cycles. However, if the system offset increases due to external causes while maximum instantaneous error signals are being generated, the reference point may never be reached. Due to these instantaneous limitations on the error signals a means of increasing the total effective dynamic range with little or no drift is highly desirable.
The problems resulting from the limitation on the maximum available instantaneous error signal may be more fully appreciated by referring to the following example. Air to ground missile systems allow an aircraft to fire an on-board missile to a designated target with great accuracy using sophisticated electronic equipment to guide the missile to the target. In one such system, known as a laser designator system, a laser beam is directed from the aircraft to the target along the target line-of-sight from the aircraft. The missile is caused to lock onto the beam and follow it to the target. For the system to function properly, the laser beam must be accurately positioned along the target to aircraft line-of-sight. Target trackers are used for this purpose.
A target tracker is a device which views the surrounding geography and functions to cause the designated target to be located at the center of the operating field of view of the target tracking sensor. For the purposes of the application, a TV tracker will be assumed, although other types of trackers, including infra-red trackers as well as laser spot trackers may also be considered. A TV tracker consists primarily of an electrooptic sensor, such as a vidicon, which views the scene within its operating field of view and generates video signals corresponding to the scene, a video processor and an error processor.
In conventional TV systems, the video signal is transmitted to a remote location and reconverted into an optical signal viewed on a suitable monitor. In a TV tracker system, the video signal is not only used to generate a view of the scene on a cockpit mounted monitor, but is also used to discriminate a suitable target within its operating field of view and cause the tracker to automatically acquire and track this target such that the target is aligned with the center of the sensor field of view and target lock-on or track is established. The tracker sensor is then properly aligned along the line-of-sight from the aircraft to the target. During the procedure, the laser beam, which has been boresighted with the TV sensor line-of-sight, follows the target movement so that it too is properly aligned with the target line-of-sight.
To effect the above described tracker operation, the tracker is provided with a video processor and an error processor. The video processor functions, generally, to condition sensor generated video signals to effectively isolate the target from the background video and to generate video signals representing target location within the sensor field of view. The video processor generated signals are applied to the error processor which generates the error signals representing the misalignment between the target location and the center of the sensor field of view. These error signals are applied to system servos which function to reposition the tracker in a direction tending to null the error signals.
As an aircraft is continuously in motion relative to a stationary or moving target, continuous repositioning of the target tracker is necessary in order to maintain the tracker line-of-sight coincident with the target to aircraft line-of-sight. However, if the maximum error signal per processing cycle is less than the actual error, the increasing error attributable to aircraft movement may outpace the error signal controlled servo movement resulting in eventual loss of lock.
A technique for effectively increasing the rate at which the tracker moves toward its reference position involves the accumulation of error signals over successive processing cycles. By accumulating the maximum error signals over successive processing cycles, a signal representing the true error is approached allowing the servos to quickly reposition toward the reference point. Previous signal accumulators used for this purpose have been analog devices. Such devices use, for example, capacitors which preclude accurate long term signal storage because such devices are susceptible to signal drift over several processing cycles. The digital accumulator of the present invention has application to such an error processor and allows error signal accumulation in a bipolar direction over several processing cycles without signal drift.
The accumulator of the present invention has application to other systems besides target trackers. Indeed, it has application to any system which requires long term signal storage with stored signal modification capability. Another system which may make use of the accumulator to which the invention pertains is an autoboresight system such as described in the co-pending patent application of Thomas and Beauregard entitled "Automatic Laser Boresighting", Ser. No. 872,196, filed Jan. 25, 1978, and assigned to the same assignee as the instant invention. The co-pending application describes an automatic laser boresighting system for automatically boresighting the laser of a laser designator system to the null point of an automatic television tracker by selectively causing the laser beam to be retroreflected to the tracker electro-optic sensor which interfaces with the television tracker video processor. The tracker locks onto the retroreflected laser spot, with the tracker error signals, in a feedback control loop, being used to control the sensor raster bias to center the sensor sweeps about the laser spot, thereby nulling the tracker error signals and achieving boresight with the laser automatically.
These are but two examples of the use of our inventive device. Various other uses will become apparent to those skilled in the art after gaining a full understanding of the invention by reading the detailed description set out hereinafter.